(EN) Regeneration of polyvalent metal ion chelated by substantially monomeric chelants for H2S removal systems is facilitated by membrane nanofiltration (15) of metal chelate solution (14). Nanofiltration removes accumulatd water and chelant degradation products and solution contaminants from the redox reaction solution (14). A purified and concentrated redox solution (17) is produced for regeneration and recycle to a gas-liquid contractor (2). Longer redox solution life results. Iron is a useful polyvalent ion.